This invention relates generally to engine pistons and, more particularly, to two-piece pistons with oil cooling of the crown portion.
Pistons for middle and large sized diesel engines, such as those having a displacement of 5 liters or more, have had considerable attention to achieve increased performance. Two-piece designs for piston assemblies have been widely adopted for such engines.
Two-piece pistons, sometimes referred to as articulated pistons, have a crown that has an upper end surface that forms part of an engine""s combustion chamber, and an outer lateral surface carrying the piston rings that run within a cylinder of the engine. The crown is typically of a high strength material, such as machined forged steel, to withstand the pressure and temperature encountered in operation.
Two-piece pistons also include a skirt below the crown that is a separate member typically of a lower strength material then the crown, but one that is lighter and more heat conductive, such as aluminum. The skirt is typically cylindrical and open, or hollow in the center, so an oil coolant can be sprayed up through the skirt against the crown. The skirt helps contain the oil to assist in heat dissipation from the piston. (The sprayed oil is also what provides lubrication for motion between the piston rings and the cylinder wall.)
For example, in U.S. Pat. No. 4,056,044 issued to Kammon, Nov. 1, 1977, a two-piece piston design is disclosed that includes a main body, or crown, and a skirt that are connected through pin bores by a wrist pin to which a connecting rod is attached. A cooling oil is directed against the interior of the crown. An annular groove in the end of the skirt that faces the crown receives at least some of the cooling oil that has drained from the crown and, due to the piston""s motion, splashes it out for further cooling and lubricating action. Such an annular groove in a skirt of a two-piece piston is sometimes referred to in the art as a xe2x80x9cshaker trayxe2x80x9d.
Other examples of two-piece pistons include those in U.S. Pat. No. Re. 34,139 issued to Cooper et al., Dec. 8, 1992, which includes further aspects of the form of the crown surface that receives the sprayed oil, and U.S. Pat. No. 4,986,167 issued to Stratton et al., Jan. 22, 1991, that includes a baffle plate to help trap coolant in an annular cooling recess, or cooling gallery, of the crown.
In the prior known two-piece pistons, the pin joint with the wrist pin connecting the crown portion and the skirt, and also joined to the connecting rod, is lubricated merely by random splashing of the jet sprayed oil or else, in engines large enough in size (such as 50 L. or more displacement), a continuous supply of oil to the pin joint through the connecting rod; for example, see above-mentioned U.S. Pat. No. 4,056,044. Random splashing provides lubrication that is uncertain and may be discontinuous. Supplying oil continuously through a passage in the connecting rod is not a very cost effective design for mid-sized engines in a range of, for example, about 5 to about 30 L. displacement.
Lack of cost effective lubrication results in limitations on engine performance and life due to component wear. There is a continuing interest in increasing the specific power (power per unit of displacement) of engines. That tends to make cylinder pressures higher and increase the loading on the piston pin joint which would have the adverse effect of reducing component life.
The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
The present invention is directed to two-piece piston assemblies with a reliable and cost effective way to provide continuous lubrication to the pin joint of pistons for a wide variety of engine sizes.
The piston assembly includes ducts or passages in the skirt near the top of the skirt, such as from the shaker trays, to the pin bores on each side of the skirt. Some oil, or other fluid coolant and lubricant, that is used to cool the piston crown falls within the ducts and is carried through the ducts to the pin joint of the union of the skirt, crown, and wrist pin for the connecting rod. The invention also increases the effective useful life of engines by reducing wear of critical components such as the piston crown and skirt, the wrist pin, and the connecting rod.
The invention retains the benefits of prior two-piece pistons and their fluid cooling techniques and provides a way to raise the specific power of engines, including diesel engines, for example, in the size range of about 5 to about 30 L. displacement, without requiring a system with oil supplied through a passage in the connecting rod.